I solved it: Use this function func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChangeContentWith snapshot: NSDiffableDataSourceSnapshot<String,NSManagedObjectID>) instead of func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChangeContentWith snapshot: NSDiffableDataSourceSnapshotReference)

I found the way to solve it. Just implement empty UICollectionViewDragDelegate & UICollectionViewDropDelegate methods. extension ReorderableCollectionViewController: UICollectionViewDragDelegate {     func collectionView(_ collectionView: UICollectionView, itemsForBeginning session: UIDragSession, at indexPath: IndexPath) -> [UIDragItem] {         return []     } } extension ReorderableCollectionViewController: UICollectionViewDropDelegate {     func collectionView(_ collectionView: UICollectionView, performDropWith coordinator: UICollectionViewDropCoordinator) {     } }

I have same issue. Also, onMove not working on macOS Big Sur Beta 6.